Process for preparing battery-grade iron phosphate by recovering ferrophosphorus slag

Abstract

The invention provides a process for preparing battery-grade iron phosphate by recovering ferrophosphorus slag, and belongs to the field of iron phosphate preparation, and the process specifically comprises the following steps: step 1, carrying out first-stage leaching to obtain first-stage leaching liquid and first-stage leaching slag; step 2, carrying out second-stage leaching to obtain second-stage leachate and graphite slag; step 3, performing primary precipitation to obtain primary precipitation mother liquor and primary precipitation white slag; step 4, performing primary dissolution to obtain a primary dissolved solution; step 5, secondary precipitation or / and secondary precipitation white slag stirring and washing: secondary precipitation is carried out to obtain secondary precipitation mother liquor and secondary precipitation white slag; stirring and washing the secondary precipitation white slag to obtain secondary precipitation mother liquor after stirring and washing and secondary precipitation white slag after stirring and washing; 6, secondary dissolution is conducted, and refined ferrophosphorus liquid is obtained; 7, synthesizing ferric phosphate to obtain an anhydrous ferric phosphate product; according to the recovery process provided by the invention, the impurity content of the prepared iron phosphate is low, the impurity content of the iron phosphate is consistent with that of the iron phosphate prepared from the raw materials, the process cost is low, various indexes of the recovered iron phosphate are qualified, and the requirement for preparing high-capacity and high-compaction lithium iron phosphate can be met.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of iron phosphate, and particularly relates to a process for recycling phosphorus-iron slag to prepare battery-grade iron phosphate. Background Art

[0002] The large-scale application of lithium iron phosphate materials in the field of electrochemical energy storage will generate a large number of retired batteries. Recycling and re-preparing them into lithium iron phosphate materials can not only achieve a closed-loop industrial chain but also avoid environmental pollution problems caused by scrapped batteries. Since the lithium iron phosphate cathode material has high requirements for impurity content, the wet recycling process with high impurity removal depth has become the mainstream in the industry. At the same time, considering that the value of lithium is higher than that of phosphorus and iron, preferential lithium extraction can maximize the lithium recovery rate to ensure economic benefits. However, due to the enrichment of impurities in the phosphorus-iron slag after lithium extraction, it is difficult to recover phosphorus and iron elements. The main phase of the phosphorus-iron slag is FePO 4 , and the valuable element therein is phosphorus element, with a content as high as 13-18%, which is equivalent to that of high-grade phosphate rock. Considering the low grade of phosphate rock in China and the dependence on imported phosphorus resources, it is of great significance to recover phosphorus instead of extracting phosphorus from phosphate rock. Therefore, developing a phosphorus-iron slag recycling process with cost advantages is the key link in the recycling of lithium iron phosphate.

[0003] The main challenge in the recycling and utilization of phosphorus-iron slag lies in how to effectively separate elements such as iron, aluminum, nickel, cobalt, manganese, and lithium therein while controlling costs, especially the separation of aluminum and iron is particularly difficult. Aluminum element mainly comes from the aluminum foil current collector and diaphragm coating in the phosphorus-iron slag. If it is not completely separated, it will be mixed into the lithium iron phosphate product with the iron phosphate precursor, which may weaken the electrochemical performance of the recycled lithium iron phosphate.

[0004] Regarding the influence of aluminum ions on lithium iron phosphate materials, previous studies have pointed out that aluminum ions can occupy both lithium sites and iron sites in lithium iron phosphate. Appropriate doping of aluminum ions can reduce the unit cell volume of lithium iron phosphate and shorten the lithium ion diffusion path, thereby improving the specific capacity and rate performance of the material. However, when the doped aluminum ions are excessive, these inert aluminum ions will reduce the specific capacity of the material.

[0005] Zhang et al. deeply analyzed the existence form of aluminum ions in the process of synthesizing iron phosphate precursor and lithium iron phosphate through phase characterization and electrochemical tests, and clearly pointed out that aluminum ions cannot really be incorporated into the lattice of lithium iron phosphate, but form inert trigonal system aluminum phosphate. This means that although the incorporation of aluminum ions itself does not directly affect the electrical performance of lithium iron phosphate, excessive aluminum ions will still reduce the overall performance of the material.

[0006] In the context of the increasingly fierce competition in the current lithium iron phosphate market, the quality requirements for lithium iron phosphate are becoming increasingly strict. Therefore, for recycled lithium iron phosphate, solving the problem of deep aluminum removal has become a technical difficulty that must be overcome before it can enter the market. In the phosphate system, the chemical properties of aluminum ions and ferric ions are similar, making it difficult for conventional selective leaching or selective precipitation to directly meet the requirements of lithium iron phosphate products. CN116588909A proposes a method for preparing lithium iron phosphate from phosphorus iron slag after lithium extraction. This method obtains a ferrous phosphate solution and graphite slag through the reduction leaching of phosphorus iron slag, and then adjusts the pH of the ferrous phosphate solution to remove aluminum and copper impurities to obtain a purified ferrous phosphate solution. After adding phosphoric acid and sulfuric acid for acidification in the purified solution, an oxidant is added to synthesize lithium iron phosphate. This type of reduction impurity removal-oxidation synthesis method has good impurity removal depth, but the large use of reducing agents and oxidants will result in too high costs. In order to reduce the cost of oxidants, CN116581415B proposes a method for the combined recovery of phosphorus iron slag after lithium extraction and waste lithium cobalt oxide. This method first uses sodium hydroxide to leach the phosphorus iron slag to obtain a sodium phosphate solution and carbon-containing iron hydroxide. The sodium phosphate solution is prepared into a phosphoric acid solution through freeze crystallization, while the carbon-containing iron hydroxide is roasted and reduced to iron powder; further, the iron powder reacts with sulfuric acid to form ferrous sulfate, and then ferrous sulfate reacts with lithium cobalt oxide by roasting to obtain lithium sulfate, cobalt sulfate and ferric oxide; this method utilizes the oxidation of ferrous oxide by lithium cobalt oxide and the reduction of ferric iron by graphite powder, saving the consumption of oxidants and reducing agents, but there are problems such as too long process flow, high energy consumption in roasting reduction and freeze crystallization, and low product value. CN114920226A proposes a short-process impurity removal process, that is, first fully mix the phosphorus iron slag with a fluoride salt and then put it into a rotary kiln for roasting to form a roasted slag containing aluminum and copper fluorides. The roasted slag is slurried and then acid is added, and by controlling the pH value, aluminum and copper complexes are selectively leached to obtain the impurity-removed phosphorus iron slag. The phosphorus iron slag can obtain a phosphorus iron solution with qualified impurities after acid dissolution and enter the synthesis process. This method has a short process flow and good impurity removal depth, but the corrosion problem of fluoride ions to equipment and subsequent treatment will greatly increase the investment and production cost of the process. Summary of the Invention

[0007] To solve the above technical problems, the present invention proposes a recovery process for phosphorus iron slag after lithium extraction in a sulfuric acid system. Its advantages are that only through simple acid leaching and precipitation operations, aluminum ions in the phosphorus iron slag can be deeply removed; at the same time, the consumption of acid and alkali in the process is relatively low. After introducing a cheap alkali source to replace the traditional sodium / ammonium alkali, it not only has a cost advantage but also the "competition" effect of metal ions can improve the iron-aluminum separation efficiency; finally, a refined phosphorus iron solution with good controllability is obtained, which is used to prepare a high-performance lithium iron phosphate precursor, breaking through a key link in the industrial recycling and reconstruction of phosphorus iron slag into lithium iron phosphate, specifically as follows: A process for recovering phosphorus iron slag to prepare battery-grade lithium iron phosphate includes the following steps: Step 1, one-stage leaching: Take lithium-extracted phosphorus iron slag / phosphorus iron slag and mix it with pure water to make a primary conditioning slurry, and heat it to 30-70°C. Add sulfuric acid to the primary conditioning slurry for acid leaching reaction. After reacting for 1-2 hours, filter to obtain a one-stage leaching solution and a one-stage leaching residue; Step 2, two-stage leaching: Take the above one-stage leaching residue and mix it with pure water to make a secondary conditioning slurry, and heat it to 30-70°C. Add concentrated sulfuric acid to the secondary conditioning slurry for acid leaching reaction. After reacting for 1-2 hours, filter to obtain a two-stage leaching solution and graphite slag; Step 3, primary precipitation: Heat the one-stage leaching solution described in Step 1 to 30-90°C, add ferrous sulfate and hydrogen peroxide to adjust the Fe / P molar ratio of the solution to 0.90-1.20, and then add an alkali source slurry for precipitation reaction for 0.5-2.0 hours. After that, filter to obtain a first-stage precipitation mother liquor and a first-stage white precipitate; Step 4, primary dissolution: Mix the first-stage white precipitate with pure water to make a third conditioning slurry, and heat it to 30-90°C. Add concentrated sulfuric acid to dissolve the first-stage white precipitate, control the pH value of the dissolution solution to 0.5-1.0. After complete dissolution, obtain a primary dissolution solution; Step 5, secondary precipitation or / and washing of the second-stage white precipitate: Secondary precipitation: After heating the primary dissolution solution to 30-90°C, add the alkali source slurry again. After reacting for 0.5-2.0 hours, filter to obtain a second-stage precipitation mother liquor and a second-stage white precipitate; Washing of the second-stage white precipitate: Take the above second-stage white precipitate and mix it with pure water to make a fourth conditioning slurry, heat it to 30-90°C and add an eluent, adjust the pH = 1.0-2.0 with sulfuric acid, react for 0.1-2.0 hours, filter to obtain a washed second-stage precipitation mother liquor and a washed second-stage white precipitate; Step 6, secondary dissolution: Put the washed second-stage white precipitate into the two-stage leaching solution obtained in Step 2 for dissolution. The reaction temperature is 30-90°C, and the reaction time is 0.1-2.0 hours. Adjust the density of the dissolution solution to 1.15-1.30 g / mL by adding the washed second-stage white precipitate. After passing the qualification, perform fine filtration to obtain a refined phosphorus iron solution; Step 7, synthesis of iron phosphate: (1). Add ferrous sulfate, hydrogen peroxide and pure water to the refined phosphorus iron solution to adjust the Fe / P molar ratio of the solution = 0.90-1.10 and the concentration of Fe to 0.5-1.5 M to obtain an adjusted solution; (2). Heat the adjusted solution to 30-90°C, add an alkali solution and react for 1.0-3.0 hours to generate an iron phosphate precipitate. Filter to obtain yellow iron phosphate, and wash the yellow iron phosphate with pure water; (3). Put the washed yellow iron phosphate into pure water to make a slurry, and add 0.1-0.8 times the molar amount of FePO 4 phosphoric acid and carry out a conversion reaction at 60-100°C for 1.0-3.0 hours; (4) Filter out the white iron phosphate material, wash and dry it to obtain iron phosphate dihydrate powder; (5) Roast the iron phosphate dihydrate powder at a temperature of 550 - 650 °C for 1.0 - 5.0 h to dehydrate and crystallize it into anhydrous iron phosphate product.

[0008] Further, in the step (1), the solid content of the primary slurry for adjustment is 10 - 25%, and the addition amount of sulfuric acid is 0.3 - 0.7 times the theoretical amount.

[0009] Further, in the step (2), the solid content of the secondary slurry for adjustment is 10 - 25%, and the addition amount of sulfuric acid is 0.8 - 1.2 times the theoretical amount.

[0010] Further, in the step (3), the Fe / P molar ratio of the first-stage leaching solution is controlled in the range of 0.90 - 1.20. The alkali source is oxides, hydroxides and carbonates of magnesium, calcium, manganese, nickel, and iron. The concentration of the alkali source slurry is 5 - 25%. The feeding method is single feeding (the first-stage leaching solution is used as the base, and the alkali solution is added to the low liquid) or combined feeding (a small amount of the first-stage leaching solution is used as the base, and the first-stage leaching solution and the alkali solution are added to the low liquid simultaneously). The pH value at the reaction end point is 1.5 - 2.5.

[0011] Further, in the step (4), the liquid-solid ratio of the third-stage slurry for adjustment is 1:1 - 10:1, and the pH value of the primary dissolution solution is 0.5 - 1.0.

[0012] Further, in the step (5), during the secondary precipitation, the alkali source is oxides, hydroxides and carbonates of magnesium, calcium, manganese, nickel, and iron. The concentration of the alkali source slurry is 5 - 25%. The feeding method is single feeding or combined feeding. The pH value at the reaction end point is 1.5 - 2.5.

[0013] Further, in the step (5), during the agitation and washing of the second-stage white residue, the solid content of the fourth-stage slurry for adjustment is 5 - 25%. The eluent is sulfuric acid or phosphate of iron, calcium, and magnesium. The addition amount of the eluent is 0.1 - 5.0% of the mass of the fourth-stage slurry for adjustment.

[0014] Further, in the step (6), after dissolution, pH = 0.5 - 1.0, and the density of the refined phosphorus-iron solution is controlled in the range of 1.15 - 1.30 g / mL.

[0015] Further, in the step (7), in the step (2), the alkali solution is one or a combination of ammonia water, sodium hydroxide, magnesium hydroxide, ammonium carbonate, sodium carbonate, and magnesium carbonate. The addition amount of the alkali solution is used to adjust the pH value of the slurry to 1.6 - 2.2; the iron phosphate yellow material is rinsed with pure water until the conductivity of the rinsing solution is 3500 - 4000 μs / m; In the step (3), the solid content of the slurry is 5 - 25%, and the temperature of the slurry is 70 - 100 °C.

[0016] Further, in step 7, in step (3), the solid content of the slurry is 5 - 25%, and the slurry temperature is 70 - 100°C.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This process uses multi-step impurity removal to separate impurity elements in phosphorus iron slag. First, it uses staged leaching for preliminary separation, leaching impurities into the first-stage leaching solution and obtaining a clean second-stage leaching solution. Second, in the precipitation process, by selectively precipitating iron phosphate, impurities are removed from the mother liquor. Since the ksp of iron phosphate and aluminum phosphate is similar, resulting in similar precipitation pH values for the two, local over-alkalinity during the reaction process will increase the aluminum content in the iron phosphate slag. Therefore, the present invention uses a weak base as the precipitant and changes the feeding method to co-feeding to maintain a stable and uniform pH value during the precipitation reaction process, avoiding the precipitation of aluminum phosphate caused by local over-alkalinity. At the same time, through the "competition effect" of ions, cations compete for dihydrogen phosphate to further reduce the precipitation rate of aluminum ions. In addition, in the second sediment white slag washing process, the present invention converts aluminum phosphate on the surface of the iron phosphate white slag into iron phosphate through the precipitation conversion reaction of the eluent, achieving multiple deep removals of aluminum and other impurity ions. Finally, by using the impurity removal method of "two sedimentations and one washing", deep removal of impurities is achieved.

[0018] 2. The iron phosphate prepared by the recovery process provided by the present invention has low impurity content, can achieve the same impurity content as that of the iron phosphate prepared from the raw materials, and has a low process cost. The comprehensive recovery cost of phosphorus iron slag can be reduced to 8000 yuan / t FePO 4 Meanwhile, the various indicators of the recovered iron phosphate are qualified and can meet the requirements for preparing high-capacity and high-compaction lithium iron phosphate.

[0019] 3. Through the two-stage leaching in the sulfuric acid system and the multiple cycles of precipitation and dissolution in the present invention, the deep removal of aluminum ions in phosphorus iron slag is effectively achieved, not only improving the removal efficiency of aluminum but also ensuring the purity of the iron phosphate product, meeting the high-quality requirements of battery-grade iron phosphate.

[0020] 4. This process does not require reduction or high-temperature roasting of phosphorus iron slag. Only by using the leaching and precipitation reactions of conventional acids and bases can phosphorus iron slag be deeply purified. Its equipment is simple, operability is strong, safety is high, and it is easy for industrial production.

[0021] 5. This process uses the methods of staged leaching and recycling of precipitation slag to dissolve amorphous iron phosphate with residual acid, greatly saving the overall acid and base consumption of the process.

[0022] 6. This process introduces a crude alkali source to replace the traditional sodium / ammonium alkali, which not only has a cost advantage but also improves the iron-aluminum separation efficiency by using the "competition" effect of ions.

[0023] 7. The ferrophosphorus recovery rate of this process is high. During the segmented precipitation process, most of the precipitation slag in the open-circuit mother liquor can be recycled and redissolved in the system, achieving a ferrophosphorus element recovery rate > 95% without increasing acid consumption. Description of the Drawings

[0024] Figure 1 SEM image of the regenerated iron phosphate prepared in Example 1; Figure 2 SEM image of lithium iron phosphate prepared from the regenerated iron phosphate prepared in Example 1. Detailed Embodiments

[0025] In the following examples of the present invention, the lithium-extracted ferrophosphorus slag is produced after the oxidation of waste lithium iron phosphate battery black powder with hydrogen peroxide. The solid content of this slag is 60%, and its composition is shown in Table 1: Table 1 Composition of the raw material ferrophosphorus slag

[0026] Example 1

[0027] (1) Take 1500 g of lithium-extracted iron phosphate slag, add 2100 g of pure water and stir to make a slurry, then add 283.7 g of concentrated sulfuric acid and react. The reaction temperature is 60 °C and the reaction time is 2 h. After completion, filter to obtain a first-stage leaching solution and a first-stage leaching residue. The first-stage leaching residue is washed with 2 L of water and then rinsed with 2 L of water.

[0028] (2) Take 600 g of the first-stage leaching residue, add 840 g of pure water and stir to make a slurry, then add 279.6 g of concentrated sulfuric acid and react. The reaction temperature is 60 °C and the reaction time is 2 h. After completion, filter to obtain a second-stage leaching solution and graphite slag. The graphite slag is rinsed with 500 mL of water.

[0029] (3) Take 0.75 L of the first-stage leaching solution and put it into a 5 L beaker. Turn on the stirring motor, with a stirring speed of 300 rpm. Set the water bath temperature to 40 °C. Add 0.325 L of 1.2 M ferrous sulfate solution and 44.0 mL of 27.5% hydrogen peroxide, stir and react for 10 min. Set the flow rate of the first-stage leaching solution to 37.5 mL / min, and continue to add 2.25 L of the first-stage leaching solution. At the same time, prepare a 20% magnesium oxide slurry as the feeding alkali solution, with an alkali solution flow rate of 10.7 mL / min, and add it to the bottom liquid together with the first-stage leaching solution. After feeding for 60 min, continue to add the alkali solution and adjust the pH of the slurry to 1.60. Filter the slurry to obtain a first-stage white precipitate and a first-stage mother liquor. The first-stage white precipitate is rinsed with 1 L of water.

[0030] (4) Take 1250 g of the first-stage white precipitate and slurry it with 1250 g of pure water, and adjust the pH to 0.8 - 1.0 by adding concentrated sulfuric acid to obtain a primary dissolution solution; (5) Put 2.0 L of the primary dissolution solution into a 3 L beaker. Turn on the stirring motor with a stirring speed of 300 rpm. Set the water bath temperature to 40 °C. Prepare a 20% magnesium oxide slurry as the feeding alkali solution with an alkali solution flow rate of 5.0 mL / min and add it to the primary dissolution solution. Feed for 60 min. After that, continue to add the alkali solution and adjust the pH of the slurry to 1.60. Filter the slurry to obtain the secondary sediment white residue and the secondary sediment mother liquor. The secondary sediment white residue is rinsed with 1 L of water.

[0031] (6) Mix 1000 g of pure water with 135 mL of the secondary leaching solution. After heating to 40 °C, add 1000 g of the secondary sediment white residue and stir and react for 1 h. Filter the slurry to obtain the stirred and washed secondary sediment white residue and the secondary sediment mother liquor. The stirred and washed secondary sediment white residue is rinsed with 1 L of water.

[0032] (7) Mix 1000 g of the stirred and washed secondary sediment white residue with 1690 mL of the secondary leaching solution. After heating to 40 °C, react for 1 h. All the iron phosphate white residue is dissolved to obtain the refined iron phosphate solution.

[0033] (8) Take 2 L of the refined iron phosphate solution, adjust Fe / P = 1.000 with ferrous sulfate and hydrogen peroxide and dilute to an iron concentration of 0.8 M. After that, heat to 40 °C, control the feeding time to be 60 min, add 20% ammonia water. After the feeding ends, react for 1 h and age for 1 h. Filter out the iron phosphate yellow material and wash it with pure water until the conductivity reaches 3500 - 4000 μs / m; then put the yellow material into pure water to make a slurry, control the solid content of the slurry to be 20%, add 0.6 times the molar amount of FePO 4 of phosphoric acid, heat to 95 °C, react for 3 h and age for 1 h, then filter to obtain the iron phosphate white material, wash it with pure water until the conductivity reaches 200 - 400 μs / m; after drying the white material, place it in a muffle furnace and calcine at 570 °C for 4 h to obtain anhydrous iron phosphate.

[0034] The physical property indexes and morphology of the anhydrous iron phosphate prepared in this example are shown in Table 2, Figure 1 as follows: Table 2 Physical Property Indexes of Anhydrous Iron Phosphate

[0035] Further, the indexes and morphology of the lithium iron phosphate prepared from this anhydrous iron phosphate are shown in Table 3, Figure 2 as follows: Table 3 Compaction Density and Electrical Performance Indexes of Recycled Lithium Iron Phosphate

[0036] From Table 2, Table 3, Figure 1 , Figure 2It can be seen that the iron phosphate recovered and prepared by this recycling process meets the raw material standard of battery-grade iron phosphate, and the lithium iron phosphate prepared from this iron phosphate shows a high level in terms of capacity and compaction.

[0037] Example 2 The difference between this example and Example 1 is that the secondary precipitation process is cancelled. The technical solution of this example is as follows: (1) Take 1500 g of lithium-extracted iron phosphate slag, add 2100 g of pure water and stir to make a slurry, then add 283.7 g of concentrated sulfuric acid and react. The reaction temperature is 60 °C and the reaction time is 2 h. After completion, filter to obtain the first-stage leaching solution and the first-stage leaching residue. The first-stage leaching residue is washed with 2 L of water and then rinsed with 2 L of water.

[0038] (2) Take 600 g of the first-stage leaching residue, add 840 g of pure water and stir to make a slurry, then add 279.6 g of concentrated sulfuric acid and react. The reaction temperature is 60 °C and the reaction time is 2 h. After completion, filter to obtain the second-stage leaching solution and graphite slag. The graphite slag is rinsed with 500 mL of water.

[0039] (3) Take 0.75 L of the first-stage leaching solution and put it into a 5 L beaker. Turn on the stirring motor, and the stirring speed is 300 rpm. Set the water bath temperature to 40 °C. Add 0.325 L of 1.2 M ferrous sulfate solution and 44.0 mL of 27.5% hydrogen peroxide, and stir and react for 10 min. Set the flow rate of the first-stage leaching solution to 37.5 mL / min, and continue to add 2.25 L of the first-stage leaching solution. At the same time, prepare a 20% magnesium oxide slurry as the feeding alkali solution, and the flow rate of the alkali solution is 10.7 mL / min. Add it to the bottom liquid together with the first-stage leaching solution. After feeding for 60 min, continue to add the alkali solution and adjust the pH of the slurry to 1.60. Filter the slurry to obtain the first-stage white precipitate and the first-stage mother liquor. The first-stage white precipitate is rinsed with 1 L of water.

[0040] (4) Take 1250 g of the first-stage white precipitate and slurry it with 1250 g of pure water, and adjust the pH to 0.8 - 1.0 by adding concentrated sulfuric acid to obtain the primary dissolution solution; (5) Take 1000 g of pure water and mix it with 135 mL of the first-stage leaching solution. After heating to 40 °C, add 1000 g of the first-stage white precipitate and stir and react for 1 h. Filter the slurry to obtain the washed first-stage white precipitate and the first-stage mother liquor. The washed first-stage white precipitate is rinsed with 1 L of water.

[0041] (6) Take 1000 g of the washed first-stage white precipitate and mix it with 1690 mL of the second-stage leaching solution. After heating to 40 °C, react for 1 h, and all the iron phosphate white precipitate dissolves to obtain the refined phosphorus-iron solution.

[0042] (7) Take 2 L of refined ferrophosphorus liquid, adjust Fe / P = 1.000 with ferrous sulfate and hydrogen peroxide and dilute to an iron concentration of 0.8 M. After completion, heat up to 40 °C, control the feeding time to 60 min, add 20% ammonia water. After the feeding ends, react for 1 h and age for 1 h, filter out the yellow iron phosphate material, and wash with pure water until the conductivity reaches 3,500 - 4,000 μs / m; then put the yellow material into pure water to make a slurry, control the solid content of the slurry to 20%, add 0.6 times the molar amount of phosphoric acid of FePO 4 Heat up to 95 °C, react for 3 h and age for 1 h, then filter to obtain the white iron phosphate material, and wash with pure water until the conductivity reaches 200 - 400 μs / m; after drying the white material, place it in a muffle furnace and calcine at 570 °C for 4 h to obtain anhydrous iron phosphate.

[0043] Compare the impurity content of the anhydrous iron phosphate prepared in this example with that prepared in Example 1 to illustrate that the secondary precipitation process of the process of the present invention can be increased or cancelled according to the required impurity removal depth. The results are shown in Table 4.

[0044] Table 4 Influence of whether there is secondary precipitation on the impurity content of recycled iron phosphate

[0045] It can be seen from Table 4 that there is no obvious difference in the contents of Cu, Ni, Co, Mn, and Ti between the iron phosphate prepared by the recovery process provided in Example 2 and that in Example 1. Increasing the secondary precipitation mainly realizes the deep removal of Al impurities. Both meet the battery-grade iron phosphate raw material standard, indicating that the secondary precipitation process of the process of the present invention can be increased or cancelled according to the Al content requirement of the product.

[0046] Example 3 The difference between this example and Example 1 is that the stirring and washing process of the secondary sedimentation white slag is cancelled, and the specific process is as follows: (1) Take 1,500 g of lithium-extracted iron phosphate slag, add 2,100 g of pure water to stir and mix into a slurry, then add 283.7 g of concentrated sulfuric acid to react. The reaction temperature is 60 °C, and the reaction time is 2 h. After completion, filter to obtain the first-stage leaching solution and the first-stage leaching residue. The first-stage leaching residue is stirred and washed with 2 L of water and then rinsed with 2 L of water.

[0047] (2) Take 600 g of the first-stage leaching residue, add 840 g of pure water to stir and mix into a slurry, then add 279.6 g of concentrated sulfuric acid to react. The reaction temperature is 60 °C, and the reaction time is 2 h. After completion, filter to obtain the second-stage leaching solution and graphite slag. The graphite slag is rinsed with 500 mL of water.

[0048] (3) Take 0.75 L of the first-stage leachate and put it into a 5-L beaker. Turn on the stirring motor with a stirring speed of 300 rpm, and set the water bath temperature to 40 °C. Add 0.325 L of 1.2 M ferrous sulfate solution and 44.0 mL of 27.5% hydrogen peroxide, and stir and react for 10 min. Set the flow rate of the first-stage leachate to 37.5 mL / min, and continue to add 2.25 L of the first-stage leachate. At the same time, prepare 20% magnesium oxide slurry as the feeding alkali solution with an alkali solution flow rate of 10.7 mL / min, and add it to the bottom liquid together with the first-stage leachate. After feeding for 60 min, continue to add the alkali solution to adjust the pH of the slurry to 1.60. Filter the slurry to obtain the first-stage white residue and the first-stage mother liquor, and wash the first-stage white residue with 1 L of water.

[0049] (4) Take 1250 g of the first-stage white residue and slurry it with 1250 g of pure water, and adjust the pH to 0.8 - 1.0 by adding concentrated sulfuric acid to obtain the primary dissolution solution. (5) Take 2.0 L of the primary dissolution solution and put it into a 3-L beaker. Turn on the stirring motor with a stirring speed of 300 rpm, and set the water bath temperature to 40 °C. Prepare 20% magnesium oxide slurry as the feeding alkali solution with an alkali solution flow rate of 5.0 mL / min, and add it to the primary dissolution solution. After feeding for 60 min, continue to add the alkali solution to adjust the pH of the slurry to 1.60. Filter the slurry to obtain the second-stage white residue and the second-stage mother liquor, and wash the second-stage white residue with 1 L of water.

[0050] (6) Take 1000 g of the washed second-stage white residue and mix it with 1690 mL of the second-stage leachate, heat it up to 40 °C and react for 1 h. All the white residue of iron phosphate is dissolved to obtain the refined iron phosphate solution.

[0051] (7) Take 2 L of the refined iron phosphate solution, adjust Fe / P = 1.000 with ferrous sulfate and hydrogen peroxide and dilute it to an iron concentration of 0.8 M. After completion, heat it up to 40 °C, control the feeding time to 60 min, add 20% ammonia water. After the feeding is completed, react for 1 h and age for 1 h, filter out the yellow iron phosphate material, and wash it with pure water until the conductivity reaches 3500 - 4000 μs / m; then put the yellow material into pure water to make a slurry, control the solid content of the slurry to 20%, and add 0.6 times the molar amount of phosphoric acid of FePO 4 Heat it up to 95 °C, react for 3 h and age for 1 h, then filter to obtain the white iron phosphate material, and wash it with pure water until the conductivity reaches 200 - 400 μs / m; after drying the white material, place it in a muffle furnace and roast it at 570 °C for 4 h to obtain anhydrous iron phosphate.

[0052] Compare the impurity content of the anhydrous iron phosphate prepared in this example with that of the anhydrous iron phosphate prepared in Example 1 to illustrate that the white residue washing process of the process of the present invention can also be increased or cancelled according to the required impurity removal depth. The results are shown in Table 5.

[0053] Table 5 Influence of Stirring and Washing on Impurity Content of Recycled Iron Phosphate

[0054] As can be seen from Table 5, the iron phosphate prepared by the recovery process provided in Example 3 still differs from that in Example 1 in terms of the Al index, indicating that the white slag stirring and washing process of the process of the present invention can also be increased or cancelled according to the Al content requirement of the product.

[0055] Comparative Examples 1-3 The operation methods of steps (3) in Comparative Examples 1-3 and Example 1 are the same, only changing the type of liquid caustic soda, and comparing the aluminum removal effects by different feeding methods. The forward feeding method is to first put all the first-stage leaching solution into the reaction kettle, and then add caustic solution to adjust the pH; the parallel feeding method is to put the first-stage leaching solution and caustic solution into the reaction kettle at the same time. The results are shown in Table 6.

[0056] Table 6 Influence of Feeding Method and Alkali Type in Precipitation Process on Impurity Removal Effect

[0057] As can be seen from Table 6, Comparative Example 1 only changed the feeding method compared with Example 1, and there was no obvious difference in the aluminum content in the first-stage white slag, indicating that the feeding method of magnesium oxide has no influence on the aluminum removal effect; in Comparative Examples 2 and 3, the parallel feeding and forward feeding experiments were carried out with sodium hydroxide, and the aluminum content in the first-stage white slag was significantly increased compared with the results in Example 1, indicating that using magnesium salt as the alkali source has a better aluminum removal effect. In addition, the parallel feeding method for sodium hydroxide can improve the aluminum removal effect.

[0058] Comparative Examples 4-6 The operation methods of steps (6) in Comparative Examples 4-6 and Example 1 are the same, only changing the slurry solid content, acid-to-material ratio and aluminum removal agent dosage in the stirring and washing process of the second-stage white slag to compare the aluminum removal effects. The results are shown in Table 7.

[0059] Table 7 Influence of Solid Content, Acid-to-Material Ratio and Aluminum Removal Agent Dosage in White Slag Stirring and Washing Process on Impurity Removal Effect

[0060] The results of Comparative Example 4 and Example 1 show that if a small amount of sulfuric acid is added during the stirring and washing process, the aluminum content of the product is lower and the aluminum removal effect is more significant; the results of Comparative Example 5 and Example 1 show that only adding sulfuric acid without adding an aluminum removal agent will weaken the aluminum removal effect; the results of Comparative Example 6 and Example 1 show that the liquid-solid ratio has no significant influence on the impurity removal effect.

Claims

1. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate, characterized in that: The steps include: Step 1, primary leaching: taking lithium ferrophosphorus slag / ferrophosphorus slag and mixing it with pure water to prepare a primary slurry, and heating it to 30-70° C., adding sulfuric acid to the primary slurry for acid leaching reaction, reacting for 1-2 hours, filtering to obtain a primary leaching solution and a primary leaching residue; Step 2, second stage leaching: take the first stage leaching residue and mix it with pure water to prepare a second slurry, and heat it to 30-70°C, add concentrated sulfuric acid to the second slurry for acid leaching reaction, react for 1-2 hours, and filter to obtain the second stage leaching solution and graphite slag; Step 3, primary precipitation: heating the first stage leaching solution described in step 1 to 30-90° C., adding ferrous sulfate and hydrogen peroxide to adjust the Fe / P molar ratio of the solution to 0.90-1.20, then adding alkaline source slurry to carry out precipitation reaction for 0.5-2.0 hours, filtering to obtain a first precipitated mother liquor and a first precipitated white residue; Step 4, primary dissolution: the first precipitated white residue is mixed with pure water to make a tertiary slurry, and the temperature is raised to 30-90°C, concentrated sulfuric acid is added to dissolve the first precipitated white residue, and the pH value of the dissolving solution is controlled to 0.5-1.

0. After the dissolution is complete, a primary dissolving solution is obtained; Step 5: Secondary precipitation and / or secondary precipitation white residue stirring and washing: Secondary precipitation: After heating the primary solution to 30-90°C, add the alkali source slurry again, react for 0.5-2.0h, and filter to obtain the secondary precipitation mother liquor and secondary precipitation white residue; Secondary precipitation white residue agitation and washing: take the secondary precipitation white residue and mix it with pure water to prepare the fourth slurry, raise the temperature to 30-90°C and add the eluent, adjust the pH to 1.0-2.0 with sulfuric acid, react for 0.1-2.0h, filter to obtain the secondary precipitation mother liquor after agitation and washing and the secondary precipitation white residue after agitation and washing; Step 6, secondary dissolution: the white residue after the second precipitation after stirring and washing is put into the second stage leaching solution obtained in step 2 for dissolution, the reaction temperature is 30-90°C, the reaction time is 0.1-2.0h, and the density of the dissolved solution is adjusted to 1.15-1.30g / mL by adding the white residue after the second precipitation after stirring and washing. After passing the test, fine filtration is performed to obtain refined ferrophosphorus liquid; Step 7, synthesis of iron phosphate: (1) Add ferrous sulfate, hydrogen peroxide and pure water to the refined ferrophosphorus solution to adjust the Fe / P molar ratio of the solution to 0.90-1.10 and the Fe concentration to 0.5-1.5M to obtain an adjusted solution; (2) Raise the temperature of the adjusted solution to 30-90°C, add alkali solution to react for 1.0-3.0 hours to generate iron phosphate precipitate, filter to obtain iron phosphate yellow material, and rinse the iron phosphate yellow material with pure water; (3) Put the washed yellow iron phosphate material into pure water to make slurry, add 0.1-0.8 times the molar amount of phosphoric acid of FePO4 and carry out conversion reaction at 60-100℃ for 1.0-3.0h; (4) filtering out the white iron phosphate, washing and drying to obtain iron phosphate dihydrate powder; (5) The ferric phosphate dihydrate powder is calcined at 550-650°C for 1.0-5.0h to dehydrate and crystallize into anhydrous ferric phosphate product.

2. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In the step 1, the solid content of the primary slurry is 10-25%, and the amount of sulfuric acid added is 0.3-0.7 times the theoretical amount.

3. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In the step 2, the solid content of the secondary slurry is 10-25%, and the amount of sulfuric acid added is 0.8-1.2 times the theoretical amount.

4. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In step 3, the alkali source is the oxide, hydroxide and carbonate of magnesium, calcium, manganese, nickel and iron, the concentration of the alkali source slurry is 5-25%, the feeding method is single feeding or combined feeding, and the reaction endpoint pH value is 1.5-2.

5.

5. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In step 4, the liquid-to-solid ratio of the tertiary slurry is 1:1-10:1, and the pH value of the primary dissolving solution is 0.5-1.

0.

6. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In step 5, during the secondary precipitation, the alkali source is the oxide, hydroxide and carbonate of magnesium, calcium, manganese, nickel and iron, the concentration of the alkali source slurry is 5-25%, the feeding method is single feeding or combined feeding, and the reaction endpoint pH value is 1.5-2.

5.

7. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In the step 5, when the secondary precipitated white slag is stirred and washed, the solid content of the fourth slurry adjustment liquid is 5-25%, the eluent is sulfuric acid or phosphate of iron, calcium, and magnesium, and the amount of the eluent added is 0.1-5.0% of the mass of the fourth slurry adjustment liquid.

8. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In step 6, after dissolution, the pH value is 0.5-1.0, and the density of the refined ferrophosphorus liquid is controlled in the range of 1.15-1.30 g / mL.

9. A process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate as claimed in claim 1, characterized in that: In step 7, in step (2), the alkali solution is one or more of ammonia water, sodium hydroxide, magnesium hydroxide, ammonium carbonate, sodium carbonate, and magnesium carbonate, and the amount of the alkali solution added is to adjust the pH value of the slurry to 1.6-2.2; the yellow iron phosphate material is washed with pure water until the conductivity of the eluate is 3500-4000 μs / m; In the step (3), the solid content of the slurry is 5-25%, and the slurry temperature is 70-100°C.

10. The process for recovering ferrophosphorus slag to prepare battery-grade ferric phosphate according to claim 1, characterized in that: In the step 7, in the step (3), the solid content of the slurry is 5-25%, and the slurry temperature is 70-100°C.

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